For improving the surface reflection characteristic of an optical element that comprises a synthetic resin, it is well known to form an antireflection film on the surface of the synthetic resin. For example, Japanese Patent Laid-Open No. 116003/1981 discloses an optical element having an antireflection film formed on a substrate of CR-39 (diethylene glycol bisallyl carbonate) resin, in which the antireflection film comprises a basic layer made of SiO2 having a thickness of 1.5 λ, a first layer made of a two-layered equivalent film constructed of a ZrO2 layer and an SiO2 layer and having an overall thickness of about 0.25 λ, a second layer made of ZrO2 having a thickness of about 0.50 λ and a third layer made of SiO2 having a thickness of about 0.25 λ that are formed in that order on the CR-39 resin substrate.
However, when an antireflection film is formed on a resin substrate through vapor deposition, the temperature resistance of the resin substrate is not as high as that of glass substrates. Therefore, it cannot be said that, for example, the heat resistance of a layer made of ZrO2 formed on a resin substrate through vapor deposition is sufficient. In addition, the heat resistance of the ZrO2 layer is apt to greatly lower with time. Optical elements of which the heat resistance of the antireflection film is not good as a whole and is apt to greatly lower with time, are often problematic in their practical use, for example, for lenses for spectacles. This is because plastic frames of spectacles are heated before lenses are fitted thereinto and the heat is conducted to the lenses having been fitted into the frames. An antireflection film of low heat resistance is often cracked owing to the difference in the thermal expansion, etc.
To solve the problem of heat resistance, for example, Japanese Patent Laid-Open No. 291502/1990 and U.S. Pat. No. 5,181,141 disclose an optical element having an antireflection film of which the high-refraction layer is formed from a vapor deposition film containing Ta2O5, ZrO2 and Y2O3, and discloses a composition for vapor deposition to form a vapor deposition film containing Ta2O5, ZrO2 and Y2O3.
The composition for vapor deposition disclosed in Japanese Patent Laid-Open No. 291502/1990 and U.S. Pat. No. 5,181,141 containing Ta2O5, ZrO2 and Y2O3, requires a relatively long period of time to form the intended antireflection film, and is therefore unfavorable in point of the working efficiency.
Especially in the field of spectacles, desired are new optical elements for plastic lenses coated with an antireflection film of which the heat resistance is good and does not lower with time.
An object of the invention is to provide an optical element that comprises a synthetic resin substrate that must be subjected to vapor deposition thereon at low temperatures and which has an antireflection film formed, and in which the heat resistance of the antireflection film is good and lowers little with time. Another object of the invention is to provide a composition that is suitable for vapor deposition and capable of forming a high-refraction layer within a shorter period of time not detracting from the physical properties intrinsic to the high-refraction layer, and to provide a method for forming an antireflection film.